The present invention relates to a sensor for monitoring the concentration of moisture and gaseous substances in the air.
In order to provide and ensure a healthy, comfortable and safe environment for both humans and machines, air conditioners and air purifiers are needed to improve the quality of ventilation. Sensor technology serving as feedback in the automatic control of the air channels is important for continuous assessment and monitoring of temperature, moisture and gaseous toxic substances.
Measuring systems of a high technical standard and reliability are already available for the determination of the humidity and gas concentration of the air. These include psychometers or dew point measuring devices for determining humidity and gas chromatography or IR-spectrometry for determining the concentration of gas. However, for economical and handling reasons, and in particular due to their complexity, these laboratory devices are not suited for monitoring in air conditioning systems. In order to determine the humidity and gas concentration in the air in a simpler and less expensive manner, various compact, ceramic sensor elements made of semiconductive metal oxides such as Al.sub.2 O.sub.3, TiO.sub.2 --V.sub.2 05, MgAl.sub.2 O.sub.4, M.sub.N WO.sub.4, MgFe.sub.2 O.sub.4, ZnCr.sub.2 O.sub.4 --LiZnVO.sub.4 or MgCr.sub.2 O.sub.4 --TiO.sub.2 were developed for moisture sensors and of SnO.sub.2, ZnO, TiO.sub.2, WO.sub.3, Fe.sub.2 O.sub.3, LaNiO.sub.2, CoO or PtO.sub.2 for gas sensors. Despite their more or less major drawbacks, these sensor systems find use in numerous practical applications.
High sensitivity, large measurement range, short response times and small size are the most important requirements for a gas sensor element. Some of these requirements are mutually compatible. However, some of them conflict with each other, making development of an ideal sensor especially difficult.
Gas sensors of the generic type described above utilize the phenomenon of adsorption of water vapor or of the various gases on the surface of a semiconductive metal oxide, which alters the physical properties of the metal oxide, in particular, its surface conductivity. Known SnO.sub.2 gas sensor are based, for example, on the phenomenon that removal of the oxygen ions adsorbed on the SnO.sub.2 surface lowers the electrical resistance of the sensor under the influence of the reducing gases. The moisture sensor is based on the principle that the resistance of many metal oxides changes logarithmically with increasing humidity. By measuring the changes in electric resistance of the metal oxide under the influence of moisture or various gases, the air moisture or gas concentration can be determined.
It is known, however, that the sensitivity of a sensor depends not only on properties of the sensitive material itself, but also on the measurement method used to determine the change in such properties. Thus, the shape of the electrical contacts ("meter electrodes") plays an important role in the effective detection of the sensor signal. The resistance of the metal oxide diminishes following the absorption of the water molecules or the reducing gases. In order to assess these changes (and thus to determine the moisture content or the gas concentration of the surroundings), two different meter electrode arrangements are presently employed in thick-film technology. Each of these arrangements has its advantages and disadvantages.
FIG. 1 shows the buildup of a prior art sensor having planar interdigitated electrodes, such as is disclosed, for example, in O. Niwa et al., Anal. Chem. 62, 1990, 447-452. First the interdigitated electrode is printed onto an AL.sub.2 O.sub.3 substrate by means of serigraphy. Then this structure is dried at room temperature and baked in an oven at high temperatures. Subsequently, the paste composed of the sensitive metal oxides is pressed onto the previously baked electrode and dried or baked.
This sensor element constructed as a planar interdigitated capacitor has the advantage that the components which are to be detected are able to reach the sensitive layer directly without encumbrance, because the finger-shaped (interdigitated) electrode arrangement is located under this active layer. Therefore this sensor has a short response time. However, a disadvantage of this type of sensor is its high internal resistance, which diminishes its sensitivity. Especially in the low water vapor or gas concentration range, the changes in the sensor resistance do not follow the concentration changes of the detected components sensitively enough. This sensor element is therefore predominantly employed in either relatively high concentration ranges or in cases in which only rapid response behavior is required.
In order to improve the sensor sensitivity, particularly in low water vapor or gas concentrations, it is necessary to reduce the internal resistance of the sensor. For this purpose, gas sensors are known, which have a "sandwich" configuration, in which the two plate electrodes are disposed parallel to each other and the intermediate space is filled with the active layer, as shown in FIG. 2. In the thick-film process, the sensor element is built up on an AL.sub.2 O.sub.3 substrate in three printing procedures as follows: the bottom plate electrode, sensor layer and top plate electrode. After printing each layer, it is dried at room temperature and subsequently baked at high temperatures.
With this configuration, and gas sensor has a low interior resistance and can therefore achieve high sensitivity even in low water vapor or gas concentrations. Because of the two large plate electrodes, it measures via a very thin active layer. However, due to the given technological conditions, despite the close mesh sieve only electrodes having layer thickness greater than the micron range can be produced in the layer thickness process. Due to the fact that the water molecules or the gas molecules first have to penetrate the thick, whole surface cover electrode in order to reach to reach the active layer, the sensor element has a long response time.
One object of the present invention, therefore, is to provide a sensor arrangement which achieves a high sensitivity, and at the same time has large measurement range, with a short response time.
Another object of the invention is to provide a sensor which achieves these goals, and is also compact.
The sensor arrangement according to the present invention accomplishes the objects described above by utilizing a "sandwich" configuration, in which a substantially planar sensitive layer is disposed between two "plates", each of which has an interdigitated structure, with opposed fingers of the interdigitated structure having an opposite polarity. This design of the electrodes results in a reduced internal resistance of the sensor, so that the sensor according to the invention possesses high sensitivity, even in the case of low concentrations of the components which are to be detected. Since the interdigitated structure does not entirely cover the surface of the sensitive layer, this arrangement also allows gas or water molecules to reach the active layer directly, thereby shortening its response time. Accordingly, a compact sensor arrangement is achieved, which has a short response time and a high sensitivity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.